1. Field of the Invention
This invention relates to an analog to digital converter for converting an analog signal to a digital signal.
2. Description of the Prior Art
Analog to digital (a/d) converters of a feedback comparing type or a successive approximation type and a non-feedback comparing type or parallel type are known. The parallel type of a/d converter determines all bit values by comparing an input analog voltage with a precisely divided reference voltages. The parallel type of a/d converter provides a high speed conversion because statically precisely divided reference voltages are employed. However, there are a lot of comparing circuits and a lot of elements in the circuit. FIG. 2A is a block diagram of a prior art parallel type of a/d converter having two-bit outputs. In this case, the number of resolution is four. There are four resistors R21, R22, R23, and R24 connected in series. Voltages at junction points between theses resistors VZ1, VZ2, and VZ3 are fed to the negative inputs of three comparators 21 to 23 as reference voltages, respectively. The output of this prior art a/d converter is determined by the values of outputs of the comparators 21 to 23 through a logic circuit. FIG. 2B shows a truth table of this prior art a/d converter. The reference voltages VZ1, VZ2, and VZ3 are determined by the resistances of respective resisters R21 to R24 and a supply voltage Vcc. This prior art a/d converter provides a high speed conversion. However, there are problems in that require increasing in resolution increases the number of comparing circuits and the number of resisters for generating the reference voltages. As a result, the conversion accuracy decreases with the rise in the resolution.